1. Field of Invention
The present invention relates generally to impedance matching circuits for power amplifiers. More specifically, the present invention relates to an impedance matching circuit for multiple band power amplifiers.
2. Description of Related Art
In the United States, cellular operating licenses have been awarded by the Federal Communication Commission (FCC) pursuant to a licensing scheme which divides the country into geographic service markets. Two cellular licenses were awarded for operating systems in each market. These two systems were initially allocated two different radio frequency (RF) blocks in the 800 MHz range. To meet the needs of a growing number of mobile subscribers, the FCC subsequently released additional blocks of spectrum in the 800 MHz range.
Despite the increase in available spectrum, demand for cellular services has continued to outpace supply. Moreover, the use of analog technology limited the capacity gains that could be achieved by conventional techniques such as cell-splitting. In response, a number of digital air interface standards were developed for providing efficient digital communication of voice, data, fax and text messages under the umbrella of "personal communications services" or PCS.
The FCC recently auctioned spectrum in the 1900 MHz range for use by PCS systems. Six frequency bands have been specified within the 1900 MHz range, with each band divided into duplex channels spaced by 30 KHz in a manner similar to the channel allocation for the 800 MHz range used by cellular systems.
Operational PCS systems are now beginning to appear in the United States. Meanwhile, existing cellular systems are continuing to operate. Thus, in many markets, there are now cellular systems operating in the 800 MHz range and PCS systems operating in the 1900 MHz range. Mobile subscribers who desire to receive services from both types of systems must either use two different mobile transceivers capable of operating within the cellular band and the PCS band, respectively, or, preferably, use a single "dual band" mobile transceiver which can operate in both bands. Moreover, with the advent of personal satellite communications, it is likely that in the future a mobile subscriber will wish to receive services from three or more systems using different frequency bands.
One approach to designing a multi-band mobile transceiver, such as a dual band transceiver, is to use completely separate radio hardware for the cellular band and the PCS band, respectively. However, this approach would increase the size and cost of the mobile transceiver. To minimize the size and cost of a dual band mobile transceiver, as much of the hardware used for operation in the cellular band should be reused for operation in the PCS band.
In particular, it is desirable to include only one amplifier in the dual band transceiver for amplification of RF signals in both the cellular band and the PCS band. Otherwise, two separate amplifier chains are required, which may be both expensive and inefficient.
There is a problem if only one amplifier is used, however. For the amplifier to act as an efficient power amplifier, the impedance at the output of the amplifier must be power matched to the impedance of the antenna prior to transmission. One possible solution to this problem is to provide the amplifier with separate, switched high-pass and low-pass matching networks at its output. However, the switch must be capable of handling high power, which tends to require a large, costly switch.
Another solution is to provide a broadband power matching circuit covering both desired frequency bands and having peaks at the transmit bands. Such a configuration would tend to waste bandwidth, however, when the desired match frequencies differ by an octave or more and the desired bandwidth in each band is relatively narrow. The well known Fano's Limit shows that there is a physical limitation on broadband matching when a reactive element (such as the drain-source capacitance of a transistor) is present.
Therefore, there is a need in the art for an improved matching circuit for providing a power match for a multi-band power amplifier.